Editor: Add new theme category for block types

Related: https://github.com/WordPress/gutenberg/pull/30020.

Introduces a new "theme" category to the default set to use with template-parts and other FSE blocks.

Props matveb.
Fixes #52883. 



git-svn-id: https://develop.svn.wordpress.org/trunk@50564 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/post.php

@@ -2206,6 +2206,11 @@ function get_block_categories( $post ) {
 			'title' => _x( 'Widgets', 'block category' ),
 			'icon'  => null,
 		),
+		array(
+			'slug'  => 'theme',
+			'title' => _x( 'Theme', 'block category' ),
+			'icon'  => null,
+		),
 		array(
 			'slug'  => 'embed',
 			'title' => _x( 'Embeds', 'block category' ),